This is because English people have a peculiar sense of humour, and enjoy putting their spare time into giving nicknames to footballers when they're not busy sipping their earl grey tea.
just kidding...
Nickname giving as a form of endearment is pretty common, it helps delude fans into fostering a make-believe close relationship between themselves and the players.
Which is also why fans of Arsenal/Chelsea/etc don't call our Giggs - Giggsy, Scholes, Scholesy so on and so forth. Likewise we don't call Sissoko momo or Reina pepe.
As for the Gaz/Wazza thing, I believe it has it's roots in English nickname giving.. The most famous -za guy would probably be Paul Gascoigne (Gazza). Some may call Darren Fletcher (and other darrens from other teams e.g. darren anderton) Dazza and so on, but these nicknames are not widespread.
In general the -y suffix is added to most last names, provided it can be added, and that it does not gay the name up too much.